My shower stall was leaking. I purchased a new strainer, cut a
hole in the ceiling, removed the old strainer and installed the
new one. Well, it still leaks.

After some detective works, I found that it is leaking between the
strainer and the gasket. The cross section of the system looks
like the following:

strainer - rubber sleeve gasket - 2" drain pipe

At first, I was able to see faint light coming through from
between the strainer and the gasket. No wonder it leaked. After
twisting the gasket, I stopped seeing the light but it sill leaks.

What is the trick of adjusting the gasket so it won't leak? Thanks.